hw/sd: Rename sdbus_write_data() as sdbus_write_byte()

The sdbus_write_data() method do a single byte access on the data
line of a SD bus. Rename it as sdbus_write_byte() and document it.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200814092346.21825-3-f4bug@amsat.org>

hw/sd/pl181.c

@@ -244,7 +244,7 @@ static void pl181_fifo_run(PL181State *s)
                 }
                 n--;
                 s->datacnt--;
-                sdbus_write_data(&s->sdbus, value & 0xff);
+                sdbus_write_byte(&s->sdbus, value & 0xff);
                 value >>= 8;
             }
         }